In sealing apparatus for preventing the leakage of a sealed fluid, such apparatus comprising two parts configured so as to rotate relatively to one another and so that end surfaces thereof slide along a plane, such as, for example, a mechanical seal, a balance must be struck between the two opposing conditions of seal tightness and lubrication in order to maintain seal integrity for extended periods of time. In recent years, environmental concerns in particular have led to an increase in demand for reduced friction in order to reduce mechanical damage while preventing sealed fluid leakage. Methods of reducing friction include the so-called fluid lubrication state, in which dynamic pressure is generated between sealing faces due to rotation, and the surfaces slide with a liquid film interposed therebetween. However, in such cases, positive pressure is generated between the sealing faces, so that the fluid escapes from the positive pressure portion outside of the sealing faces. Such fluid outflow constitutes leakage in the case of a seal.
Mechanical seals such as that shown in FIG. 5, in which dynamic pressure is generated between sealing faces via rotation, are known in the art (“prior art”; see, for example, patent document 1). In the prior art shown in FIG. 5, a plurality of radial grooves 32R, 32L for generating dynamic pressure during rotation is provided in the circumferential direction of a sealing face 31 of a mating ring 30 constituting one of a pair of sliding parts, with a dynamic pressure-generating groove 32 comprising tapering surfaces 33R, 33L tapering in opposite directions being formed following the circumferential direction so that the boundary between one pair of radial grooves 32R, 32L is in a trough formed by the tapering surfaces, and a dam 34 being formed at the boundary so as to separate the radial grooves 32R, 32L.
As shown in FIG. 5(b), when the sliding parts rotate relative to each another, the pressure in the radial direction groove 32R, which lies in the upstream direction of a sealed fluid flow G, decreases, creating negative buoyancy, and the wedge effect of the tapering surface 33L in the radial direction groove 32L on the downstream side of the dam 34 increases pressure, creating positive buoyancy. At this time, the action of the dam 34 decreases the negative pressure and increases the positive pressure, creating a net positive pressure and allowing a strong buoyancy to be obtained.
However, the dynamic pressure-generating groove 32 of the prior art has a shape for creating a dynamic pressure effect, and does not have an element for controlling seal integrity. Thus, there is the problem that, while dynamic pressure is generated by the dynamic pressure-generating groove 32 when the mating ring and a seal ring constituting the sliding parts rotate relative to each other, the generation of the dynamic pressure causes the fluid film to thicken, and the sealing faces of the mating ring and the seal ring break contact, so that, while sliding resistance decreases, leakage increases.
In addition, the dynamic pressure yielded by the dynamic pressure-generating groove 32 according to the prior art is not generated unless the rotating shaft reaches a certain degree of rotational speed. There is also the problem that, for this reason, sufficient quantities of sealed fluid cannot be introduced between the sealing faces during the period from when rotation begins until dynamic pressure is generated, leading to reduced lubrication and increased torque, in turn leading to the problems of seizing, vibration, noise, and the like being generated and sliding properties becoming unstable.
Examples of prior art in which dynamic pressure-generating grooves are provided in order to prevent wear during sliding part rotation are known (for example, see patent document 2), but, because these examples lack an element for controlling seal integrity, like the prior art described above, they have the problem of increased leakage.